I love cellular automata, so I love Make Magazine's Cellular Automata Video Synthesizer Kit.
With its retro screen resolution and bright, 8-bit era colors, it's the perfect generator for the pixels that could, forming bizarre and complex patterns derived from simple predetermined rule sets. It has three knobs to control the artificial life formulae and a "go" button to start the machine rolling.
It also produces music, but, to be honest, I'm hoping that's not procedurally-generated, because procedurally-generated music sucks.
Source code and instructions [critterandguitari]
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